r - ggplot2: How to use same colors in different plots for same factor

Question

How can I pin the same color to a value in diffent plots?

Say I have two data.frames df1 and df2:

library(ggplot2)
library(gridExtra)

set.seed(1)
df1 <- data.frame(c=c('a', 'b', 'c', 'd', 'e'), x=1:5,  y=runif(5))
df2 <- data.frame(c=c('a', 'c', 'e', 'g', 'h'), x=1:5,  y=runif(5))

When plotting them using c as color-indicator I get the same five colors.

g1 <- ggplot(df1, aes(x=x, y=y, fill=c)) + geom_bar(stat="identity")
g2 <- ggplot(df2, aes(x=x, y=y, fill=c)) + geom_bar(stat="identity")
grid.arrange(g1, g2, ncol=2)

But I want that same values of c get the same color.

Answer 1

You can set your own fill scale using scale_fill_manual. I create a named vector with colors and different values of "c".

dd <- union(df1$c,df2$c)
dd.col <- rainbow(length(dd))
names(dd.col)  <- dd

Then :

g1 <- ggplot(df1, aes(x=x, y=y, fill=c)) + 
  geom_bar(stat="identity") +
  scale_fill_manual("Legend", values = dd.col)
g2 <- ggplot(df2, aes(x=x, y=y, fill=c)) + 
  geom_bar(stat="identity") +
  scale_fill_manual("Legend", values = dd.col)
grid.arrange(g1, g2, ncol=2)